1. Field of the Invention
The present invention relates to a cordless telephone apparatus, particularly to a cordless telephone apparatus that, when during the course of a telephone call there has been a brief interruption of power to the internal circuits of the remote unit (such as caused by the internal battery of the remote unit losing contact with the battery contacts, for example), enables the telephone call to be resumed over the same pair of transmit-receive radio channels established for the call.
2. Description of the Prior Art
There are known cordless telephone systems consisting of: a base unit that is driven from a commercial power source and has a wired connection to a public telephone line circuit; and a remote (handset) unit driven by an internal primary or secondary battery source; and two radio channels of different frequencies used to provide a two-way or bidirectional communication link between the base unit and remote unit. Recently systems in which a single base unit can handle multiple remote units have become popular. Multiple base unit configurations are also possible.
FIG. 5 shows a conventional type of cordless telephone apparatus arrangement consisting of one base unit 90 and multiple remote units 80. As the multiple remote units 80 all have the same internal circuit configuration, only one remote unit 80 is shown.
Because the base unit 90 does not get moved around much, it can be driven from a commercial power source. The commercial power circuit 29 shown in the drawing can encompass an AC adapter or the like that provides a rectified output to power the internal circuits of the base unit. On the other hand, the convenience of the remote unit 80 resides in its portability, which means it has to rely on a primary or secondary battery 19 as its power source. In view of the fact that primary batteries have a limited current capacity and secondary batteries have a limited service life in terms of the number of charge-and-discharge cycles, it has to be easy for a user to replace the battery, whether the battery 19 is a secondary or a primary battery. As described later, this has a bearing on the problem to be solved by this invention. Here, however, we will continue with the general description of the arrangement of FIG. 5.
Nowadays the main control circuit set that provides integrated control of the internal circuits in both the remote unit and the base unit is usually a microcomputer. In this system the transmit-receive circuit 12 in the remote unit 80 is controlled by an internal microcomputer 16 and the transmit-receive circuit 22 in the base unit 90 is likewise controlled by an internal microcomputer 26, to thereby effect two-way communication of the various control and voice signals between antennas 11 and 21, via a pair of radio channels (pair of radio frequencies). A calling signal indicating a call coming in via the public telephone service line, an originating signal indicating an outgoing call from the remote unit 80, dial signals accompanying the operation of a keyboard (not shown) provided on the remote unit 80, and an end code signal indicating the termination of a call are some control signals. Other control signals include the out-of-range signal described later. Generally these are all digital signals that follow a prescribed format and which are appropriately modulated in accordance with the respective modulation systems of the transmit-receive circuits 12 and 22 for transmission via the antennas 11 and 21.
The microcomputers 16 and 26 also each perform other control functions specific to the associated remote unit 80 and base unit 90. In the case of the remote unit 80, one such control function relates to battery saving or battery conservation operation. This consists of supplying power to the transmit-receive circuit 12 being switched on and off by the transmit-receive power control circuit 13 under the control of the microcomputer 16. In the waiting-for-call state or the stand-by state the supply of power from the battery 19 to the transmit component of the transmit-receive circuit 12 is switched off and just the receive component of the transmit-receive circuit 12 is intermittently operational by power from the battery 19 being switched on and off at prescribed intervals under the control of the transmit-receive power control circuit 13. When multichannel access (usually abbreviated to "MCA") system is used for the two-way radio communication between multiple remote units 80 and the base unit 90, the receive component of the transmit-receive circuit 12 is only able to scan the plurality of usable receiving channels (sending channels, if viewed from the base unit end) when it is receiving power from the battery under the control of the transmit-receive power control circuit 13. When a fixed-channel system is used, the receive component is tuned to the preset channel frequencies. If the scanning by the MCA system or the tuning by the fixed-channel system produces no signal detection and demodulation output by the receive component of the transmit-receive circuit 12, the power to the receive component is switched off by the transmit-receive power control circuit 13 under the control of the microcomputer 16, and is switched back on after a prescribed time period.
On the other hand, when the receive component of the transmit-receive circuit 12 does show a signal detection and demodulation output, the demodulated information is decoded by the microcomputer 16, and when this information is found to contain the specific address data, meaning (the ID (identification) number data), assigned to its own remote unit 80, in order to continue the following operation and start actual voice communication, the transmit-receive power control circuit 13 continues to supply battery power to both the receive component and the transmit component of the transmit-receive circuit 12 until the microcomputer 16 has transmitted an end code via the transmit component of the transmit-receive circuit 12, in response to the user either operating a switch (not shown) to terminate calling or placing the remote unit 80 on its charger cradle (also not shown). Voice information demodulated during the operation of the receive component reaches the user via the telephone receiver (not shown) of the remote unit and. Likewise, voice information spoken into the telephone transmitter is modulated by the transmit component and transmitted from the antenna 11, via the established sending channel (receiving channel, viewed from the base unit).
In response to the user either lifting the remote unit 80 from the charger cradle (not shown) or operating a switch (also not shown) to originate a call, an originate call signal is transmitted by the microcomputer 16 and followed by the transmission of a dialing signal to establish a state of communication with the other party via a public telephone line, after which power is supplied continuously to the transmit-receive circuit 12 until an end code has been transmitted by the microcomputer 16. This is also the case when the other party's line is busy and the person using the remote unit, therefore, intentionally terminates the attempted communication.
In contrast, because the power to the base unit 90 comes from the commercial power line 29, under the control of the microcomputer 26, a transmission power control circuit 23 supplies power to the transmit component of the transmit-receive circuit 22 only when it has to transmit (this is done mainly to prevent unwanted EMF radiation when transmission is not necessary, rather than to save power), but the receive component of the transmit-receive circuit 22 receives a constant supply of power and waits for an originate call signal from the remote unit 80 by continuously scanning the prescribed plurality of receiving channels (sending channels, if viewed from the remote unit 80 end), when an MCA system is used, or, if a fixed-channel system is being used, by remaining tuned to the fixed channel. The base unit 90 is connected to a public telephone line via a line switching circuit 25 that includes a hybrid circuit or the like. Under the control of the microcomputer 26 this circuit 25 forms at least the same line switching control as a normal wired telephone set.
A conventional cordless telephone apparatus also incorporates, in the remote unit 80, a voltage detection circuit 15 that monitors the voltage of the replaceable battery 19. If the output voltage of the battery 19 drops below a prescribed level during a call, a signal indicating this is output by the voltage detection circuit 15 whereby, via an interface (not shown), the microcomputer 16 activates a light-emitting diode or other such light-emitting means and/or causes the telephone receiver to emit a warning sound to inform the user that the battery 19 needs charging. While it depends on the apparatus concerned, if the battery is not charged and the voltage continues to drop, the microcomputer 16 goes into a stop mode in which all functions are shut down. After the battery 19 has been replaced or sufficiently recharged, a power-on reset is applied to restart operation.
Microcomputers 16 and 26 are equipped with respective volatile random-access memories (RAMs) 18 and 28, and non-volatile programmable read-only-memories (PROMs) 17 and 27, provided externally or on the same chip. Nowadays memories 17 and 27 are much more likely to be electrically-erasable PROMs (EEPROMs). The EEPROM in the remote unit 80 contains control information and at least the ID number assigned to that remote unit 80, while the EEPROM in the base unit 90 contains at least the ID number data of each of the multiple remote units that can be used with that base unit. PROM as used here and throughout refers to a non-volatile memory means, including when some or all such PROMs are EEPROMs.
In line with regulations governing the use of cordless telephone systems in Europe in particular, the radio link between base unit and remote unit, and the telephone line connection, has to be broken if the received field strength in the remote unit has fallen below a prescribed value and remains low for a prescribed time after the user has been alerted by a warning light or a warning tone from the telephone receiver. The remote unit 80 therefore includes an out-of-range detection circuit 14, a field strength detection circuit incorporated in accordance with the constitutional principles of known noise or carrier squelch circuits. If (via this out-of-range detection circuit 14) the microcomputer 16 detects that the received field strength falls below 30 dB.mu.V/m, for example, after a prescribed period, for example one second, the microcomputer 16 alerts the user by emitting a signal that causes a warning means (not shown) to emit a warning lasting a prescribed period, for example 10 seconds (either continuously or in one-second pulses with a one-second separation). The microcomputer also causes an out-of-range signal 40 to be appropriately modulated in accordance with the format shown in FIG. 4 and transmitted from the antenna 11 to the base unit 90. Usually, if the received field strength does not recover within the prescribed signal transmission period (such as the above 10 seconds), the microcomputer 16 issues an end code via the transmit-receive circuit 12. If the received field strength does recover in time the microcomputer 16 stops the transmission of the out-of-range signal and resets the related signal duration timer.
The out-of-range signal 40 is handled in the same way as the other control signals described above, and uses the data format shown in FIG. 4, for example. This begins with communication data discrimination information 41 consisting of a 20-bit set of hexadecimal digital numerical values. This forms a code word indicating that communication information follows. This is followed by a 24-bit code word that forms that unit's ID number data 42, and an 8-bit set of connection channel data 43 indicating the designated radio channels currently established between base unit and remote unit (and not, therefore, needed if a fixed-channel system is being used rather than the MCA system). This is followed by an 8-bit set of out-of-range data 44 for indicating the transmitted communication data is the out-of-range signal 40. With the same data format applying in principle with respect to other control data, out-of-range data 44 is substituted for other control information. In other words, cordless telephone systems that use the out-of-range signal 40 have out-of-range data 44 in the form of a designated code word as one of the various types of communication information that conforms to the usual data format.
When the out-of-range signal 40 is received by the transmit-receive circuit 22 of the base unit 90 and decoded by the microcomputer 26, and as a result of no interruption occurring for the duration of the above prescribed time period an end code signal is received from the remote unit 80, the microcomputer 26 uses the line switching circuit 25 to open the line circuit.
The regulations relating to the out-of-range signal 40 thus apply only to the remote unit 80. However, the present inventor has already proposed a system in which the received field strength is also monitored at the base unit end. As in the arrangement at the remote unit end, if there is a drop in the received field strength at the base unit an out-of-range signal 40 is sent from the base unit to warn the user. After this warning has continued for a prescribed period of 10 seconds, for example, the telephone line circuit is opened by the line switching circuit 25. In a cordless telephone apparatus using a 900 MHz band, for example, in which the radio channel pair is formed by two waves separated by 45 MHz, even if the operating environment is such that an adequate received field strength can be ensured for the remote unit receiving channel, it is because the remote unit sending channel (base unit receiving channel) field strength remains quite low. With the idea that it was not enough to detect the field strength only with respect to the remote unit receiving channel, as shown in FIG. 5 the base unit 90 end was provided with the same type of out-of-range detection circuit 24 as the remote unit 80 to, therefore, form an arrangement whereby the same process is used to transmit an out-of-range signal 40 conforming to the data format of FIG. 4 from the base unit end when it is detected that the received field strength at the base unit 90 end has fallen below a prescribed value. The constitution of this out-of-range signal 40 from the base unit is advantageously applied in the preferred embodiment of the invention described below.
A representative arrangement of a conventional cordless telephone apparatus and a brief outline of the operation thereof have been described in above with reference to FIGS. 4 and 5. Changing a remote unit battery in most of the relatively early cordless telephone systems involved having to disconnect and connect special connector means, and the battery itself was usually a battery pack. This was to ensure a secure electrical connection between the battery 19 and the various circuits in the remote unit. In recent years, however, there has been growing demand for batteries that can be easily changed by anyone, and this has led to an increase in the use of battery holders with spring contacts, and no regard to whether batteries are primary or secondary.
Inasmuch as the ease of use of such battery holders has made them familiar to everyone, their use by a cordless telephone apparatus can be said to make it a better system. However, there is also a problem with battery holders, which is that if someone using a remote unit accidentally drops the unit on the floor or bangs it against a piece of furniture or the like, the impact can cause a spring contact to lose contact with a battery terminal, cutting off power to the circuits and halting all operations. Even if battery contact is restored almost immediately, it is too late to enable the call to be resumed. This is a frequent problem with systems which use a microcomputer as the control circuitry.
The problem is not limited to such accidents. When a user is making a call via a remote unit, and the voltage detection circuit 15 detecting a drop in the unit voltage activates a signal warning the user no matter how quickly the battery is replaced, it is too slow to prevent the power to the circuits being cut, breaking the call connection. JP-A Hei-2-177647 discloses a means designed to counter this problem. Specifically, in order to change the battery it is necessary to remove a battery holder cover. A means (substantially an electromechanical contact means) of detecting when this cover is removed is therefore incorporated, and when this means detects the removal of the cover, a changing-battery signal is from the remote unit to the base unit, which responds by putting the line connection on hold. When the replacement battery is then inserted, a signal signifying this is from the remote unit to the base unit, which then releases the line it has been holding. This means that detects the cover has been opened is described in the claim as a means that detects the battery has been removed. Substantially, however, it is interpreted as being limited to a cover open detection means, as described in the embodiments of the disclosure. However, as the supply of power to the circuits of the remote unit obviously stops the moment the voltage is removed, removing the battery prevents the operation of the very means that is supposed to detect the removal, and of the circuit that processes the detection signals.